1. Field of the Invention
The present invention relates to a keyboard unit and, particularly, to a keyboard unit suitable for use in a keyboard input device.
2. Description of the Related Art
A keyboard unit for a keyboard generally comprises a housing, membrane switches, each having upper and lower electrodes which are incorporated in this housing and opposed to each other with a predetermined space therebetween, key tops, each supported by lifting units in such a manner that it can move vertically with respect to the housing, and rubber springs each of which is arranged below each key top and can be elastically deformed.
As for the operation of this keyboard unit, when the key top is moved down in defiance of the elastic force of the rubber spring by depressing, the rubber spring is reversed in the course of the movement of the key top to generate a click touch and the membrane switch is turned on.
When the above pressure applied to the key top is removed, the key top supported by the lifting units rises by the return force of the reversed rubber spring and the membrane switch is returned to an OFF state in the course of the movement of the key top.
In recent years, demand for a thin keyboard unit has been becoming greater and greater along with the popularization of notebook-sized small and thin portable personal computers. However, a conventional keyboard unit having the above lifting units has limitation in reducing its thickness because the height of the rubber spring becomes higher than required to generate a click touch.
It is therefore an object of the present invention to provide a keyboard unit which can make an operator feel a large stroke of a key top and can give a sure click touch though it is thin.
According to a first aspect of the present invention, there is provided a keyboard unit which comprises key tops, membrane switches whose contact points are changed by depressing the key tops, piezoelectric elements which can be curved and deformed by the application of voltage and a base plate for mounting the piezoelectric elements, wherein each of the piezoelectric elements is driven by changing the contact point of the respective membrane switch and this driving is transmitted to the respective key top through a stem.
According to a second aspect of the present invention, there is provided a keyboard unit wherein a domed reversible return spring is interposed between each key top and each membrane switch and depressed and reversed by depressing the key top to change the contact point of the membrane switch.
According to a third aspect of the present invention, there is provided a keyboard unit wherein a stem made of an elastic material is provided on the rear of each key top and elastically deformed and contracted by depressing the key top after the return spring is reversed by depressing the key top with the stem.
According to a fourth aspect of the present invention, there is provided a keyboard switch wherein the piezoelectric element is arranged to form a predetermined space below the membrane switch and bent toward the space when the contact point of the membrane switch is changed by depressing the key top.
According to a fifth aspect of the present invention, there is provided a keyboard unit wherein the space is formed by a spacer arranged between the piezoelectric element and the base plate or a recess formed in a portion where at least the center portion of the piezoelectric element of the base plate is located.